Electronic – 7 segment binary to hex

7segmentdisplayhex

I'm trying to display a 4-bit binary number on a 7 segment LED as a hexadecimal number (0-F). I have an assortment of 7400 series ICs including the 7447. But that one only works for BCD (0-9). The 7400 series doesn't seem to have a hex to 7-segment decoder and I don't have one on hand.

So I figure I'd have to build my own. The datasheet for a 7447 comes with the internal circuitry but I couldn't find a similar circuit for hex to 7-segment. I did K-diagrams for each LED segment by hand but the terms are rather large, much larger than for BCD.

Does anyone have a finished circuit for this that I can check my work against?

Does anyone have an optimized circuit for this that takes advantage of common subterms? There aren't many duplicate terms (like A0 & ~A1 & A2) but maybe using (A0 & A2) & ~A1 would allow sharing the (A0 & A2) subterm and overall reduce the gate count.

Or maybe some tricks to use NAND, NOR or XOR gates for some parts?

I don't care about different path length or races in the circuit as it's only going to drive LEDs and should be far too quick to see any of that.

Best Answer

Find what museum is missing their 74xx logic ICs and return them. Then get a small microcontroller and do all this in a single chip. As a bonus, you'll be ready for other advanced projects from the late 1980s and beyond.

Related Solutions

7-segment driving circuit not working as expected

From a first look and a glance at the datasheet, generally speaking this seems fine. But the Arduino isn't exactly nippy at changing those outputs and while they are changing (with LE low), the outputs will also be changing. Could that be what you mean by "flashing" display? I'd guess that the display isn't really flashing per-se, it's just changing slowly enough for you to see it as a big mess.

I would suggest setting pin 8 HIGH to begin with - which will block any changes from the BCD pins - then changing all the BCD outputs and finally setting pin 8 LOW to reflect those changes. Then delay and loop. Something like this maybe:

Important note from your code: In C at least, & has lower precedence than !=. So the expression:

i & 0x1 != 0

resolved as if you'd typed:

i & (0x1 != 0)

which is:

i & 1

That's a trap I've seen before. Use brackets to clearly state what you mean and always do so where & and | is concerned, especially if there's an == or != nearby. I've changed it below:

  ...
  for( int i = 0; i < 10; ++i )
  {
    digitalWrite( 8, HIGH );
    digitalWrite( 2, ((i & 0x0001) != 0) ? HIGH : LOW );
    digitalWrite( 3, ((i & 0x0002) != 0) ? HIGH : LOW );
    digitalWrite( 4, ((i & 0x0004) != 0) ? HIGH : LOW );
    digitalWrite( 5, ((i & 0x0008) != 0) ? HIGH : LOW );
    digitalWrite( 8, LOW );
    delay( 1000 );
  }

7 Segment display binary to decimal

It is not recommended lo leave logic inputs floating. You should use a pull-down / pull-up resistor (see Spehro Pefhany's comment).

schematic

^{simulate this circuit – Schematic created using CircuitLab}

This circuit inputs low level by default (switch open). If you press the switch then it inputs high level.

schematic

^{simulate this circuit}

This circuit inputs high level by default (switch open). If you press the switch then it inputs low level.

Related Topic